A time division transmission scheme including a TDD scheme time-divides the same frequency and separately uses the divided frequencies for transmission/reception. That is, the time division transmission scheme divides one frame for transmission/reception and performs bidirectional communication through one frequency. Since a TDD system employing such a scheme separately performs transmission and reception through the same frequency according to a predetermined time period, a high speed RF switch for transmission/reception switching is essentially necessary.
Since the RF switch must perform a high speed switching operation, a switch using a semiconductor device such as a pin diode and a Field Effect Transistor (FET) is used instead of a mechanical switch. However, it is difficult to use such a switch using a semiconductor device as a switch for high power because a semiconductor is less tolerant to high power.
In other words, when high power is applied to the switch, much heat is generated. If sufficient protection against heat is not ensured, the switch may eventually be broken. Further, a RF switch developed to be tolerant to high power must have a separate refrigerator, etc., and thus becomes very expensive. In addition, it is difficult to manufacture the RF switch. Therefore, the RF switch is primarily used for military purposes only.
In order to solve this problem, the TDD system has employed a method for fixedly separating transmission signals from reception signals by using a circulator instead of the RF switch. However, in the case of using a circulator, it is difficult to sufficiently ensure isolation for blocking transmission signals during a reception interval. When an antenna enters an open state due to the occurrence of a problem in the antenna during the transmission of transmit power, transmission signals enter a receiver and thus abnormality may occur in the system, or the quality of reception signals may significantly deteriorate. Further, transmission Passive Intermodulation Distortion (PIMD) occurs and affects the radio wave quality of other communication providers.